1. Field of the Invention
The present invention relates to a high-performance thermal head (300 dpi or more) mounted on a thermal transfer printer, for example, and a bonding connection method therefor.
2. Description of the Related Art
The thermal head includes a thermal storage made of a highly insulating material, such as glass, a plurality of exothermic bodies generating heat by turning on electricity, a plurality of individual electrodes individually and electrically connected to the exothermic bodies, a head substrate having a common electrode electrically connected to the entire exothermic bodies, a plurality of driving ICs arranged for each of the individual electrodes and individually controlling electrification/non-electrification of the plurality of individual electrodes, and a driving IC substrate having a plurality of IC-electrode pads to be connected to the individual electrodes corresponding to the plurality of driving ICs, so that a printing material is printed by abutting the exothermic body heated via the common electrode and the individual electrode to the printing material wound around an ink ribbon and a platen roller.
In the thermal head described above, the individual electrode and the corresponding IC-electrode pad can be wire-bonded together with a connection wire. Since the IC-electrode pads are arranged at an interval smaller than that of the exothermic bodies, an AU wire about 25 μm in diameter is used as the connection wire. The connection with the wire bonding generally uses a capillary 40′ for bonding a thermal head that, as shown in FIGS. 13A to 13C, includes an insertion hole 41′ for inserting a connection wire 31′ thereinto and a circular pressure end surface 42′ for pressurizing the connection wire 31′ inserted into the insertion hole 41′. Specifically, the connection wire 31′ and the IC-electrode pad or the individual electrode are connected together by abutting the circular pressure end surface 42′ of the capillary and the connection wire 31′ together on the IC-electrode pad or the individual electrode so as to apply ultrasonic vibration thereon in this state.
In recent years, the head has been further improved in performance (recording density), so that the number of the exothermic bodies, i.e. the number of printing dots, formed on one head substrate is increased without changing the planar area of the head substrate or with reducing the area. Since the numbers of the individual electrodes and the IC-electrode pads are also increased in association with this, the individual electrodes and the IC-electrode pads are stepwise arranged in a plurality of rows (staggered arrangement). When such a staggered arrangement is adopted on the driving IC substrate, the greater the number of the printing dots is, the area of the substrate (size in width) is also increased, so that the number of the driving IC substrates formed from a single wafer is reduced, which makes it costly. In order to reduce cost by increasing the number of the driving IC substrates formed from one wafer, it is desirable that the pitch of the IC-electrode pads be reduced as small as possible so that a plurality of the IC-electrode pads be arranged in a row substantially in parallel with the arranging direction of the exothermic bodies. Similarly, it is also preferable that the pitch of the individual electrodes be decreased as small as possible.
However, when the pitches of the individual electrodes and the IC-electrode pads are reduced, the wire diameter of the capillary used in bonding the connection is restricted, so that the connection area of the connection wire, to which the individual electrode and the IC-electrode pad are bonded with the capillary, becomes smaller. The individual electrode and the IC-electrode pad are made of Al and formed on a hard ceramic substrate or glass. In order to directly bond the connection wire on this Al, an alloy of Al and Au (eutectic phase made by the thermal diffusion of Al) must be sufficiently developed by breaking an oxide film on the surface of Al. Hence, if the connection surface area to the connection wire is not sufficiently ensured, the connection strength is not sufficiently obtained, so that the specific electric resistance fluctuates due to unstable contact and the reliability is reduced (the presence of breaking of wire). For increasing the connection surface area, the energy required for forming the alloy of Al and Au may be increased by increasing the force pressurizing the connection wire. However, if the force pressurizing the connection wire is increased, an upstanding portion of the connection wire is reduced in thickness (sectional area), so that the tensile strength is decreased. In such a manner, the relationship between increase in the connection surface area of the connection wire and that in thickness of the upstanding portion of the connection wire is incompatible, so that both the increases are difficult to be simultaneously satisfied.